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In QAM (Quadratrue Amplitude Modulation) is combination of (ASK) and (PSK) , its alternative name (APK) ( amplitude phase keying ) ..
The general analytical expression for QAM is
Where the phase term will have M discrete values ,typically given by
MiM
ii ,.....3,2,1,
*2
8-QAM modulation : Each quadrate has the variation of phase and amplitude ,which also represents the data is only difference with 1 bit
The block diagram of 8-QAM modulation :
The block diagram of 8-QAM modulation consists of :
1- Bit splitter which convert series data to parallel date, in our case there are 3 bits, therefore the output data will be 1/3 of the input signal .* The output of bit splitter will be in three channels (I,Q,C), each channel will transmit one bit .
2- 2 to 4 level converter to convert the (I,Q,C) output data to its corresponding analog voltage.
For example (Q,I,C) =(0,0,0) = -0.541QAM = -0.541 *cos(wt) - 0.541 *sin(wt)QAM = .765 sin(wt-135)
I/Q C Output voltage
0 0 -.541
0 1 -1.304
1 0 .541
1 1 1.304
Similarly , for other codes ,we can obtain the corresponding analog voltage
Q I C Amplitude Phase
0 0 0 .765 -135
0 0 1 1.848 -135
0 1 0 .765 -45
0 1 1 1.848 -45
1 0 0 .765 135
1 0 1 1.848 135
1 1 0 .765 45
1 1 1 1.848 45
8-QAM demodulation :
The output of balance demodulator:
After the balance demodulator there are comparators and data combiner .
The performance of M-array QAM :
As M increase the data rate increase and probability bit errorincrease for a given Eb/No , but with constant bandwidth.(why?)
The probability bit error of QAM modulation:
Practical Parts :
Use (HCS-8000-03) module to modulate pseudo data with 500 mV and 20 KHz sine wave .
Demodulate the modulated output signal of the first part use the QAM demodulation.
